On 10/02/2023 08:55, David Woodhouse wrote:
On Fri, 2023-02-10 at 00:50 +0100, Thomas Gleixner wrote:
On Thu, Feb 09 2023 at 20:32, Usama Arif wrote:
On 09/02/2023 18:31, Thomas Gleixner wrote:
first_cpu = cpumask_first(cpu_online_mask);
smp_call_function_single(first_cpu, mtrr_save_fixed_ranges, NULL, 1);
So why is this relevant after the initial bringup? The BP MTRRs have
been saved already above, no?
I will let David confirm if this is correct and why he did it, but this
is what I thought while reviewing before posting v4:
- At initial boot (system_state < SYSTEM_RUNNING), when mtrr_save_state
is called in do_cpu_up at roughly the same time so MTRR is going to be
the same, we can just save it once and then reuse for other secondary
cores as it wouldn't have changed for the rest of the do_cpu_up calls.
- When the system is running and you offline and then online a CPU, you
want to make sure that hotplugged CPU gets the current MTRR (which might
have changed since boot?), incase the MTRR has changed after the system
has been booted, you save the MTRR of the first online CPU. When the
hotplugged CPU runs its initialisation code, its fixed-range MTRRs will
be updated with the newly saved fixed-range MTRRs.
I knew that already :) But seriously:
If the MTRRs are changed post boot then the cached values want to be
updated too.
I had previously only done smpboot time measurements for the whole
patchset, but I tested the patchset without this commit and it doesn't
make a difference to smpboot time as its negligable work to read those
MTRR MSRs into mtrr_state.fixed_ranges.
This commit is also independent of parallel smp bringup, similar to
reusing timer calibration so I think it could be considered as a
separate patchset if needed. I will post the next revision without this
commit, but here is my view on MTRR save/restore (which shouldn't matter
for the next revision...).
If the MTRR changes on a running system, there might be a bug during
hotplug in the original code that handles MTRR? which is also carried
over in this patch.
From what I can see, MTRR is only saved+restored during initial boot,
hotplugging CPU and __save/__restore_processor_state() (used in creating
image for hibernation, suspend, kexec...). So if for e.g. in a running
system (that has not hibernated, suspended, kexeced), if MTRR for CPU0
(first_cpu) changed post-boot and CPU3 is hotplugged, only MTRR for CPU3
is updated and CPU0 and CPU3 will hold the same value, while the rest of
the CPUs will have the older first-boot value? This behavior will happen
with or without this patch. I think this is what Thomas is referring to
above when he says that the cached values want to be updated? But the
issue is present in the original code as well.
Thanks!
Usama
They are, aren't they? The only way we come out of mtrr_save_state()
without calling mtrr_save_fixed_ranges() — either directly or via
smp_call_function_single() — is if they've already been saved once
*and* system_state < SYSTEM_RUNNING.
I suppose we could make that clearer by moving the definition of the
mtrr_saved flags inside the if (system_state < SYSTEM_RUNNING) block?
@@ -721,11 +721,20 @@ void __init mtrr_bp_init(void)
*/
void mtrr_save_state(void)
{
int first_cpu;
if (!mtrr_enabled())
return;
+ if (system_state < SYSTEM_RUNNING) {
+ static bool mtrr_saved;
+ if (!mtrr_saved) {
+ mtrr_save_fixed_ranges(NULL);
+ mtrr_saved = true;
+ }
+ return;
+ }
+
first_cpu = cpumask_first(cpu_online_mask);
smp_call_function_single(first_cpu, mtrr_save_fixed_ranges, NULL, 1);
}
